1. Field of the Invention
The present invention relates to a mini environment type sample transfer unit, particularly to a sample transfer unit suitably used for a critical dimension scanning electron microscope (CD-SEM) which measures a pattern on wafer or a photomask in manufacturing and inspection processes of a semiconductor integrated circuit.
2. Description of the Related Art
Conventionally, in semiconductor integrated circuit manufacturing process, a clean room has been used to avoid generation of dust and foreign matters. However, recently a clean environment retaining method called a mini environment type has been used instead of the clean room. In the mini environment type, only a space where cleanliness is required is kept at a locally high cleanliness level.
In the semiconductor integrated circuit manufacturing, mini environment type transfer and storage means are standardized. For example, there are well-known SEMI (Semiconductor Equipment and Materials Institute) standard, SMIF (Standard of Mechanical Interface), and FOUP (Front-Opening Unified Pod).
In a semiconductor integrated circuit manufacturing process, the mini environment type is used in not only a wafer handling process but also a process of handling a photomask which is of an original edition of photolithography. In the photomask, a light shielding pattern is formed on a quartz glass having a plate size of 152×152×6.35 mm called 6025. The photomask wafer has the pattern size four times a pattern size on the wafer. Recently, with the progress of the micro pattern, foreign matters and defects of several hundreds nanometers level are not permitted even in the photomask.
An example of a transfer unit used in critical dimension scanning electron microscope (CD-SEM) for measuring the pattern size on the semiconductor integrated circuit manufacturing photomask will be described. When compared with a silicon wafer, the photomask has a heavy weight per sheet, and frequently the single plate processing is performed to the photomask. Therefore, the photomask is transferred one by one. Usually, a SMIF pod is used to transfer the photomask. The SMIF pod is a mini environment type container to store one photomask and the inside of the SMIF pod is kept at a high cleanliness level.
A load port is used to deal with the SMIF pod. The load port is a mini environment type SMIF pod dealing mechanism. In the mechanism of the load port, the SMIF pod is opened and closed in a clean environment, the photomask is taken out from the SMIF pod, and the photomask is inserted into the SMIF pod. The photomask is taken out from the SMIF pod and transferred to a load lock chamber by a robot provided in the transfer unit. The load lock chamber is evacuated to a vacuum level. When the vacuum level in the load lock chamber reaches a predetermined value, a door between the load lock chamber and a sample stage of the critical dimension scanning electron microscope (CD-SEM) is opened to transfer the photomask from the load lock chamber to the sample stage. The pattern size of the photomask is measured with an electron beam. After the pattern size is measured, the photomask is transferred in the reverse path and stored in the SMIF pod again.
The series of the processes, i.e., the open and close of the SMIF pod, the transfer of the photomask, the evacuation of the load lock chamber, the transfer of the photomask to the sample stage, the pattern size measurement of the photomask, and the transfer of the photomask back to the SMIF pod are controlled by a computer and visually displayed by a display device. The series of the processes is performed in a locally clean environment by the mini environment type.
Usually, only one photomask can be stored in the SMIF pod. Accordingly, the SMIF pod on the load port cannot be changed until the pattern size measurement is ended for one photomask. Usually, an operator manually disposes the SMIF pod on the load port. It is necessary for the operator to change the SMIF pod at predetermined time intervals. In order to enhance the work efficiency, it is necessary for the operator to immediately change the SMIF pod to the next SMIF pod when the photomask is returned to the SMIF pod.
There is also well known a SMIF pod which stores a plurality of photomasks. However, the SMIF pod usually used can store only one photomask. Accordingly, in the case of the use of the SMIF pod which stores a plurality of photomasks, it is necessary to replace the photomasks. It is necessary to perform the photomask replacing work in an environment having at least the substantially same cleanliness as that in the SMIF pod. Accordingly, the working efficiency is lowered in the case of the use of the SMIF pod, which can store a plurality of photomasks.